Various power supply apparatuses in which many secondary batteries such as alkaline secondary batteries and nickel-metal hydride secondary batteries are connected serially have been proposed. Such power supply apparatuses are now attracting a great deal of attention especially as driving power sources of electric vehicles. Since a high voltage of 200-300 V is required as a driving power source of an electric vehicle, each of such power supply apparatuses is provided with a battery unit including about 200 secondary batteries connected serially. In such a power supply apparatus, the battery unit is charged by an external power source and discharged to an external load such as a motor. Usually, the electrical power unit is provided with monitor and control means for monitoring a state of each battery to prevent both overcharging and overdischarging. The monitor and control means stops the charging when the battery unit voltage reaches a predetermined upper limit thereby to prevent overcharging in charging, and the monitor and control means stops the discharging when the battery unit voltage drops to a predetermined lower limit thereby to prevent overdischarging in discharging.
Regardless of the battery type, a deterioration of a secondary battery including a decrease in battery capacity and an increase in internal resistance thereof progresses acceleratively as charge/discharge cycles are repeated. Further, if many secondary batteries are used, the deterioration of battery badly affects other batteries. According to the control as described above, for example, a secondary battery whose capacity is reduced due to the deterioration is further deteriorated since it is apt to be overdischarged. In addition, heat due to the increase in internal resistance of the secondary battery causes a drop in output or deterioration of other secondary batteries. Also a normal secondary battery deteriorates and generates heat thereby increasing the surrounding temperature and even when charged fully, may be overcharged, if the charging efficiency of the secondary battery around it drops.
If many batteries are to be used, a temperature difference among batteries, for example, causes the deterioration of batteries. In addition to the facilitation of the deterioration due to high temperature, a difference in charging efficiency or self-discharge characteristics among the batteries caused by a temperature difference also causes the overcharging or the overdischarging of the batteries. Further, even when every secondary battery is used under the same condition, a difference in original characteristics among respective batteries such as capacity, charging efficiency and self-discharging characteristics also becomes a factor is the deterioration.
Therefore, various means to suppress such accelerative deterioration in battery characteristics have been proposed. For example, the Official Gazette of Unexamined Japanese Patent Publication No. Hei 6-231805 has proposed a method in which a voltage of a battery unit composed of a plurality of batteries connected serially is monitored and a voltage of each battery is also monitored, then a charge upper limit voltage is corrected to be lower and a discharge lower limit voltage is corrected to be higher when a variance in battery voltage becomes large.
As shown in FIG. 4, a power supply apparatus 30 comprises a battery unit 32 for supplying an electric power to a load 33 such as a motor, and control and monitor means 34 for controlling and monitoring the battery unit 32.
A charging operation of the power supply apparatus proposed in the above-mentioned gazette will be explained with reference to a flow chart shown in FIG. 5. In charging, the monitor and control means 34 controls charging of the battery unit 32 performed by a power source 31 according to a state of the battery unit 32 while monitoring the state of the battery unit 32. When the charging is started, the monitor and control means 34 makes the power source 31 supply a current to the battery unit 32 while monitoring a voltage of each battery included in the battery unit 32. In step 301, the monitor and control means 34 compares the variance (.sigma..sub.0) in voltage among the batteries with a preset value (D.sub.0). At this time, if the .sigma..sub.0. is smaller than the D.sub.0, the monitor and control means 34 judges the battery unit 32 is being normal and keeps the supply of current from the power source 31 to the battery unit 32 until the charged amount (C) of the battery unit 32 reaches a predetermined upper limit capacity (C.sub.u) (in step 302). On the other hand, if the .sigma..sub.0 is not smaller than the D.sub.0, the monitor and control means 34 changes the charging amount used to stop charging from C.sub.u to a corrected upper limit capacity (C.sub.uc) which is smaller than C.sub.u (in step 303). At this time, the battery unit 32 is charged up to the C.sub.uc, then the charging is finished.
According to this method, the accelerative deterioration of the battery can be prevented, but this method includes a problem wherein the full performance of the battery is not brought out since batteries with no deterioration are not charged fully.
Next, a discharging operation of this power supply apparatus will be described with reference to a flow chart in FIG. 6. When discharging is started, the monitor and control means 34 supplies an electric power from the battery unit 32 to the load 33 while monitoring a voltage of each battery. At this time, it compares a variation (.sigma..sub.1) in the voltage among batteries with a preset value (D.sub.1) in step 401. If the .sigma..sub.1 is smaller than the D.sub.1, the monitor and control means 34 judges the battery unit 32 as being normal and keeps the battery unit 32 discharged to the lower limit capacity (C.sub.1) that is the original discharging lower limit (step 402), then stops the discharging. If the .sigma..sub.1 is over the D.sub.1, however, the monitor and control means 34 changes the capacity value at which the discharging is stopped from the C.sub.1 to a corrected lower limit capacity (C.sub.1c) which is larger than the C.sub.1 (in step 403). And accordingly, the battery unit 32 is discharged to C.sub.1c and the discharging is finished.
According to this method, it is possible to prevent an accelerative deterioration of batteries as same in a charging operation, but it is impossible to carry out the full performance of the battery. In addition, since it is impossible to eliminate a memory effect of a battery whose performance has dropped, the variance in capacity among the batteries cannot be eliminated substantially.